The present invention is directed to non-evaporable getter alloys for the sorption of hydrogen. In particular the invention deals with non-evaporable getter alloys having good properties of hydrogen sorption at relatively low temperatures.
Many applications in the field of industry or research require for their correct working a hydrogen-free environment in a closed container; the space inside the container may be either kept under high vacuum conditions or filled with an atmosphere of a given gas (or gas mixtures). Examples of industrial applications in which hydrogen is detrimental are evacuated jackets for thermal insulation (e.g. in thermal bottles, also known as “thermos”, or solar collectors), owing to the high thermal conductivity of this gas; some types of lamps, in which the presence of hydrogen in the filling gas generally results in the variation of the operating physical parameters (such as the lighting voltage); or X-ray generating tubes. The processes for manufacturing these devices comprise a step of container evacuation and possible filling thereof with the desired gas, but whenever a high vacuum or a hydrogen-free gas is produced, mechanisms exist which cause hydrogen to re-enter the system; these mechanisms are mainly the degassing of the container walls and the hydrogen permeation across these walls from the external atmosphere toward the container, thus leading to problems in the correct operation of said devices. Owing to the same mechanisms, hydrogen also represents the main contribution to the residual pressure in ultra-high vacuum (UHV) systems, such as the particle accelerators employed in the research field.
To remove these hydrogen traces it is known to employ non-evaporable getter materials (known in the field as NEGs), i.e. materials being capable of chemically fixing molecules of hydrogen as well as of other gases such as water, oxygen and carbon oxides. The getter materials are generally metals of the III, IV and V transition groups of the Periodic Table or alloys thereof with titanium-based and, particularly, zirconium-based alloys. These materials and their use for sorbing gases from evacuated spaces or from inert gases are well known and described in a number of patents, such as U.S. Pat. No. 3,203,901 (zirconium-aluminum alloys), U.S. Pat. No. 4,071,335 (zirconium-nickel alloys), U.S. Pat. No. 4,306,887 (zirconium-iron alloys), U.S. Pat. No. 4,312,669 (zirconium-vanadium-iron alloys), U.S. Pat. No. 4,668,424 (zirconium-nickel-Rare Earth alloys with the optional addition of one or more other metals), U.S. Pat. No. 4,839,085 (zirconium-vanadium-E alloys, wherein E is an element selected among Fe, Ni, Mn and Al), and U.S. Pat. No. 5,961,750 (zirconium-cobalt-Rare Earths alloys).
In particular, as far as hydrogen sorption is concerned, the use of yttrium or solid mixtures containing the same is also known. U.S. Pat. No. 3,953,755 discloses the use of this element (protected by thin layers of other metals) at the inside of discharge lamps. British Patent Specification GB 1,248,184 discloses the use of solid mixtures or intermetallic compounds of yttrium with other metals for sorbing hydrogen in various applications. This patent requires that yttrium is anyhow present in form of a separate phase in a sufficient quantity to accomplish the gettering function, so that the getter properties of the compositions according to that patent are essentially the same as those of pure yttrium. This characteristic can also be ascribed to the fact that with many of the metals listed in the patent (zirconium, titanium, niobium, hafnium, molybdenum, tantalum, tungsten and vanadium) yttrium does not form compounds nor alloys, whereas with other metals (aluminum, beryllium, cobalt, copper, iron, magnesium, nickel, manganese and zinc) yttrium only forms intermetallic compounds but not alloys (see the book “Constitution of Binary Alloys”, First Supplement, edited by R. P. Elliot, McGraw-Hill, (1965)). The yttrium quantities there indicated are, however, such that in the composition this element is ensured to be in excess with respect to the quantity that could be bound in form of intermetallic compounds, whereby at least a portion thereof remains in a form of pure metal.
Finally, International patent application WO 03/029502 discloses yttrium-vanadium and yttrium-tin compositions being rich in yttrium; also in this case the hydrogen sorption properties of the material are essentially those of pure yttrium. The function of the metals added to yttrium in these two last documents is mainly that of enhancing the hydrogen sorption by the getter.
NEG materials show a sorption behavior with respect to hydrogen different from that towards other gases. While for most gases the chemical sorption by these alloys is irreversible, the sorption of hydrogen by NEGs is an equilibrium process reversible as a function of the temperature: hydrogen is efficiently sorbed at relatively low temperatures (under 200-400° C., according to the chemical composition of the material), but it is released at higher temperatures. The equilibrium features of these materials in sorbing hydrogen are generally represented graphically by means of curves giving, at different temperatures, the equilibrium pressure of hydrogen over the alloy as a function of the hydrogen concentration in the NEG material.
Another feature of the NEGs is that, in order to accomplish their function, they generally require a treatment of initial thermal activation at temperatures that can vary between about 300° C. and up to about 900° C. during a time comprised between few minutes up to several hours depending on the material composition.
Advantageous features for a NEG material to be employed for hydrogen sorption are a low hydrogen equilibrium pressure and a low activation temperature.
Among the previously cited NEG materials those with the best features of hydrogen sorption (low equilibrium pressures) are the zirconium-aluminum alloys, the zirconium-cobalt-Rare Earths alloys and yttrium. Among these materials the zirconium-aluminum alloys have a high activation temperature. In order to carry out a good activation of these alloys in a not excessively long time it is necessary to activate them at temperatures higher than 700° C. This feature makes them not suitable for any application, such as when the chamber to be kept free from hydrogen has glass walls, e.g., thermos or some lamps. Yttrium and compositions of British Patent GB 1,248,184 (which, as seen before, are functionally the same as pure yttrium) only work well if kept at relatively high temperatures of more than about 600° C. The zirconium-cobalt-Rare Earths alloys require lower temperatures of activation and operation, but have worse properties of hydrogen sorption (particularly the equilibrium pressure) than those of yttrium.